1. Field of the Invention
The present invention relates to an image conversion method for converting a digital image obtained by a scanning unit into a converted digital image. The image conversion method includes the steps of acquiring statistics for at least some pixels of the digital image, extracting characteristic values of a background color using the acquired statistics, operating a selection of pixels in the digital image, each pixel in the digital image being sequentially set as a target pixel, and converting the selected pixels.
2. Description of Background Art
Documents to be scanned often have large portions of non-marked areas representing the background of the image. However, when the scanner is not well-calibrated or when the non-marked areas in the documents are far from white, as may be the case with old yellowish papers or blue prints, the digital image obtained by scanning such documents presents background areas which are not white. Therefore, in the raster image file obtained by scanning such a document, the background areas are represented by non-white pixels. Background correction algorithms are applied to the digital image obtained by a scanning unit in order to detect background areas and correct the pixels in such areas, most of the time to white pixels. When a raster image is to be printed, a white pixel in the raster image file is translated into a dot ‘not to be printed’. The benefits of background correction algorithms are, among other things, ink or toner saving when the converted image is printed. In addition, a more easily compressible and more rapidly viewable image when it is stored and/or viewed on a computer can be obtained.
An image conversion method of the above type is known from U.S. Pat. No. 6,323,957. It is attempted to eliminate undesirable background dots which give rise to noise within a document obtained by scanning of an original. With the known method, an original document is scanned by a scanner and transformed into a stream of pixels. Statistics for at least some pixels of a pixel stream generated by the scanner are acquired in order to determine a background color of the document. Incoming pixels of the scanned document are compared with the value of the determined background color. Pixels found to be ‘near’ background color values are selected and are converted. Pixels found ‘far’ from the background color values are not selected (meaning that they are left unchanged). The pixels found between ‘near’ and ‘far’ are selected and are converted in a linear manner. For each incoming pixel, the color distance between said incoming pixel and a reference pixel corresponding to the background color is calculated and the result of the calculation is used as a criterion to select or not select said incoming pixel. If the incoming pixel is selected, it is converted. A significant disadvantage is that the converted images obtained by the known method often present a speckling effect. Specifically, in substantially uniform colored areas, the presence of many white points may occur. In background areas, the presence of many colored points may occur as well.